cmd/protoc-gen-go: add link to devsite for protoc-gen-go
cmd/protoc-gen-go: add link to devsite for protoc-gen-go

Change-Id: I67a0897df42e177782ec76bb307ea2334f86975e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220596
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go